Large-Volume Direct Injection Techniques

The sample can be introduced into either a flash vaporizer or directly onto the end of the column. The best technique depends on the application, the sample, the column type, and whether the column is heated isothermally or by temperature-programming. Instantaneous vaporization of the sample on injection is the usual method of ensuring a reproducible retention time and maintaining good efficiency of separation. This approach, however, is unsatisfactory for samples containing heat-sensitive compounds (commonly encountered in biomedical applications). Samples that are very dilute and require a large volume to be injected also cause problems. 

There are three injection techniques for introducing a sample into a GC equipped with a capillary column split injection, splitless injection, and on-column injection. Split injection is the most often used injection technique. When a certain amount of FAME sample (1 to 3 ll) is introduced into the GC injector that is normally set at a temperature much higher than the boiling point of the solvent, the solvent vaporizes instantly in the carrier gas and creates a large volume of gas that contains all of the injected FAME in it. The carrier gas that contains the FAME is then divided into two streams from the injector one is directed onto the column, and the second is vented to the atmosphere, clearing the sample out of the injection chamber momentarily. This way, only a limited amount of sample is introduced into the column, to avoid column overloading, and injection time is short, to avoid peak broadening. 

Injection ports for packed columns are aligned so that the sample can be deposited on a heated surface just before the column or directly on the end of the column. In the first instance, the injection port is heated above the boiling point of the sample in order to get rapid volatilization, but for on-column use, the injection port is kept at column temperature. On-column injection is usually preferred because there is less chance of decomposition and the sample is not exposed to a high injection port temperature. Remember also that a typical GC analyte will have a retention ratio of 0.25 or much less. This means that 75% or more of it is sorbed in the stationary phase, and that is where the on-column technique puts it—in the stationary phase. It is probably for this reason that on-column injection is so efficient. The use of large volumes of solvents will wash... 

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